1. Field of the Invention
The present invention relates generally to digital xray imaging and, more particularly, to a method of digital mammography that uses dual-energy apparatus and methods for separating a single human breast x-ray image into component images, each representing a single physical substance.
2. The Prior Art
Typical clinical mammography is performed by using x-ray films. Recently, large format two-dimensional semiconductor digital x-ray detector arrays have been available. Currently, regardless of the technology, all of the image information is contained in a single image, acquired through the use of x-rays with a single energy-spectrum. The human breast is comprised of three major substances, the lean tissue, the fat tissue, and microcalcification deposits, and each pixel of such a single image contains a mixture of all three, plus a random scatter component. The size of the contribution from each component is not known in current mammography.
It is well established that the role of the random scatter signal in an x-ray imaging is interference and distortion. The scatter blurs the image, reduces the image contrast, and degrades the image quality. The contribution of scatter in mammography is generally as high as 30% to 60% of the total image signal.
It has also been established by breast cancer research, that the cancer cells have an x-ray attenuation coefficient very close to that of glandular duct which has an average composition of typical lean tissue. Thus, only the lean tissue component provides useful information for the diagnosis of cancer. The direct value of the fat tissue information for the diagnosis is very small. At the same time, the fat tissue or adipose tissue occupies as much as about 50% of the total breast volume and is closely mingled with the lean tissue. Thus, in mammography, the fat tissue signal also acts more as an adverse factor, imposing a strong, irregular pattern to mask the useful information.
The image signal of lean tissue is used for direct determination of breast cancer or other pathological changes. The microcalcification component is an important reference because it has been found that most breast cancer patients have a large amount of microcalcification in breast tissue.
Because of the inability to separate the four basic signal components, the capability of current x-ray imaging for cancer diagnosis has been essentially limited.
The present invention is based on the dual-energy x-ray imaging method disclosed in U.S. Pat. Nos. 5,648,997 and 5,771,269 and U.S. patent application Ser. No. 09/025,926 (the Chao disclosures). In terms of the prior art, the applicability of dual-energy x-ray imaging to mammography is discussed in several journal articles. The method and apparatuses used in these articles are essentially different from the present invention. For example, Chakraborty et al. (An Energy Sensitive Cassette for Dual-Energy Mammography, 16(1) Medical Physics 7 (January/February 1989)) designed a double film cassette to use a "dual-energy subtraction method" for enhancing image contrast. Johns and Yaffe (Theoretical Optimization of Dual-Energy x-Ray Imaging with Application to Mammography, 12(3) Medical Physics 289 (May, June 1985)) conducted experiments using linear detector arrays and linearization approximations for dual energy decomposition. First, the hardware between these references and the Chao disclosures is different; the detectors of the prior art are either film cassettes, linear detector arrays, or stimulable phosphor plates. Using an intermediate digitizing method to convert the analog image data either from x-ray films or from stimulable phosphor plates into a digital format is essentially a semi-quantitative imaging system. The present invention uses two-dimensional large format integrated semiconductor detectors for direct high-accuracy quantitative imaging. Secondly, the dual-energy decomposition in the prior art is invariably based on linearization methods, while the present invention is based on directly solving a dual-energy fundamental equation system in its original form without relying on linearization. Thirdly, the present invention uses imaging hardware which combines scatter removal capability and dual-energy data acquisition in one system.